We usually call it Maximum Flash Sync Speed because of our concern when using flash, but it's not about the flash — it’s instead a limitation of the focal plane shutter, and actually is a Maximum Shutter Sync Speed when trying to sync with the flash. The flash must wait to be triggered until the shutter is fully open to pass it (Sync meaning the flash triggers when the shutter is fully open). The shutter motion takes a little time to be fully open, so the flash has to wait until the far edge of the frame becomes open. Another problem is that for fast shutter speeds, the opening edge of the focal plane shutter starts closing before the far side is fully open. That “early start” of closing means the shutter is never fully open, which causes an unexposed black band in the picture (photos below), which is a limit on the fastest shutter speed that can sync with flash. The faster that the shutter design can fully open, the faster the maximum sync speed possible (before it starts closing). This description is continued below, and Wikipedia has some nice illustrations of the action of these two curtains.
Almost all of the DSLR cameras now have focal plane shutters covering the film or sensor plane like a curtain. The focal plane (FP) shutter is in the camera body instead of in each lens. Focal plane shutters have been used in the better 35 mm film cameras since the first Leica in 1925 (and focal plane shutters are even older than that), because they provide superior timing of fast shutters (and not being in the lens greatly simplify the lens mechanisms). The really big deal about focal plane shutters is their precise accuracy. They use two curtains across the sensor, called first or front curtain, and the second or rear curtain. The first curtain opens and then the second curtain closes. The two curtains use the same drive motor, same gears, same curtain speed, for any shutter speed, be it 30 seconds or 1/8000 second. No complicated speed switching or gearing. One simple precise mechanism, with one mode and one curtain speed. One curtain opens the shutter, and the second curtain closes it.
The focal plane shutter exposure duration is determined only by the delay of the second curtain starting time. Today, the timing between the curtains is controlled by a quartz crystal clock (instead of a complex mechanical assembly using springs and gears), providing very fast and accurate 1/8000 second speed, and also a very slow 30 second speed (both extremes are very difficult for a coiled spring mechanism — those older mechanical shutters only could be used between about one second and maybe 1/1000 of a second, or more typically 1/500 second). This focal plane system is great for interchangeable lenses, because it only needs one good shutter in the body, instead of a shutter in every lens. And this is a better and more accurate shutter, but focal plane shutters do have the limitation of maximum shutter sync speed with flash.
The duration of electronic flash is extremely brief, but the shutter must be fully open first, so that all areas of the frame are exposed simultaneously. At focal plane shutter speeds faster than maximum sync speed, the shutter is not fully open, and we see a black unexposed band at the top or bottom of our flash pictures (because focal plane shutters are vertical travel today). If you rotate your camera to portrait orientation, you will see this dark band down the one long side. If the camera has communication with the flash to know it is present, the camera likely will not allow a shutter speed faster than maximum sync speed.
Beginners becoming aware of this sync limitation are often unhappy that they cannot use a really fast shutter speed with flash. But even if they could, a faster shutter speed like 1/1000 second would truncate the maximum power level flash duration which lasts longer, so closing while still bright, terminating the exposure, resulting in much less exposure. Which might still might be usable in some situations, but it would make automatic exposure calculation about impossible (because the flash power could not represent the flash exposure). The Nikon D1 camera did have a fast shutter with 1/500 second sync in 1999, but this feature was abandoned in future models.
So flash sync speed is an issue, often limited to around 1/200 second shutter speed in many DSLR cameras which use the better focal plane shutters. We show examples of that issue here.
The Nikon D300 and D8xx (and the D7xxx family too) use faster shutters with higher life expectancies, and these better grade shutters offer Maximum Sync Speed at 1/320 second, shown here. Their specs say 1/250 second, but they work well at 1/320 second. There is a warning in the manual about faster than 1/250 sec, which I interpret to mean that 1/320 might cut off a little of a long flash duration, which could apply to some studio flash, or to a speedlight if using its slower full power level.
All images are always inverted in the camera, the lens projects images upside down on the sensor, so the actual travel is the opposite direction (and note that the focal plane shutters on these two camera models are moving in different arbitrary directions). The illuminated area is the traveling open slit width, which is fully open over full frame only up to maximum sync speed. At faster shutter speeds, the rear curtain begins closing sooner to make the open slit be more narrow. The speedlight flash is fast enough to stop the moving slit. These pictures simply show that open slit.
Many camera shutters only sync flash at up to about 1/200 second. Any time you see a problem like this, just reduce your shutter speed, so that it does not exceed maximum flash sync speed (in your camera's specs for Flash Sync Speed, also called X sync). This is what maximum sync speed is, it is a shutter situation, about when that shutter is fully open. Maximum sync speed is NOT a property of the electronic flash. Instead it is simply the fastest focal plane shutter speed that the front curtain is fully open, and the rear curtain has not started to close (slit is open wide enough to provide a full open frame for the flash). But sometimes radio triggers add a delay which causes the same maximum sync problems (to diagnose, try flash with and without the radio trigger. Also it might just be time for new batteries in the radio trigger).
The band may not be fully black, you might see something in it if there is enough ambient to show it, but the band area will block the flash. The darker band may not always even be noticeable in bright sun, the bright ambient will illuminate that area pretty well. FWIW, if this were a full frame camera shooting into a cropped format, that might crop a narrow dark band off, so it might get away with a slightly faster shutter speed than if shooting full frame. The two shutters appear about the same speed here, but the full frame shutter has to travel 1.5x greater distance than the cropped APS-C shutter.
Both cameras use the same 24-70 mm lens at the same 70 mm focal length. So yes, this example also shows the difference in the cropped and full frame view with the same lens. The cropped view appears as a telephoto illusion (frame cropped smaller by smaller sensor, but then also necessarily enlarged more to view at the same size again), and the full frame view is more wide angle (not cropped).
These samples were a Nikon SB-800 speedlight flash (fast flash - 1/17800 second at 1/32 power, which stops the shutter travel too). The flash exposure is not affected by shutter speed, where we are able to see it. However, this is regular flash mode, and is NOT Auto FP flash mode (next page). This flash was not in the hot shoe, and was instead triggered with a PC cord — so that the camera was unaware any flash was present, so then it allows shutter speeds faster than maximum sync speed.
If the Nikon camera can recognize that a flash is present, it will prevent exceeding maximum sync shutter speed with a flash, even in camera manual mode. If the flash is off camera or the brand has no communication with the camera (no flash known present), then no shutter speed is prevented.
An undetected flash (off camera on radio trigger or PC Sync cable, or a non-system flash (off brand or studio flash, etc) will not prevent this issue from occurring. I used a PC sync cord off camera to be able to take the too-fast shutter speeds above. The point is, with the exception of HSS flash mode, the shutter speed with flash should not exceed the maximum shutter sync speed of a focal plane shutter (mostly on DSLR).
First, the simplest concept of a shutter is a large solid sheet that covers the sensor. It is opened by pulling it back towards say the left side of the frame to expose the sensor. Then it is closed again by returning it back to the right to cover the frame again. But for a fast exposure time, the time to open and close is a significant part of the total exposure. Specifically the right side of the frame is exposed more than the left side (opened first and closed last). The shutters that are actually mounted in lenses (called leaf shutters) are more complex, being several interleaving blades designed to all open and close more quickly, but still not in zero time.
A focal plane shutter is in the camera body, being two moving shutter curtains stacked in front of the sensor (like two roller window blinds). One curtain opens to expose the sensor to light from lens, and after a timed duration, the second curtain follows in the same direction, closing to cover it. The same mechanism going the same speed operates both curtains. That makes the exposure duration (and effective shutter speed) simply be the delay between the two curtain start times (electronically timed today).
But to implement even faster shutter speeds, the second curtain starts closing before the first is fully open, tracking together to create a narrow slit between the two curtains, the open slit moving across the frame (actually down the frame today, the shorter travel is faster). This narrow slit reduces the exposure under it (of time duration equal to the difference between the two curtains start times), to be a faster shutter speed, which can easily be very fast, but a narrow slit cannot expose the full frame area from a fast flash of light. That problem is due to shutter speed exceeding the Maximum Flash Sync Speed.
A few descriptions of focal plane shutters are available. The Wikipedia link has an interesting section on history and improvements.
A very interesting photograph (a classic) using a focal plane shutter is this 1912 picture by Jacques Henri Lartigue. This is NOT a flash picture, there are no dark bands, the daylight ambient was continuous, constantly present during the exposure. The shutter slit's travel motion can cause distortion of a moving subject... different parts of the frame are exposed by a moving narrow slit at slightly different times, when things may have moved as the slit is moving too.
In this picture, the shutter slit was moving up in the picture (down in the camera, picture is inverted). The spectators and roadside poles are leaning left because this 4x5 inch camera was being panned right, following the race car. Their feet were exposed earlier and their heads later (moving left in the frame as the camera panned by), at slightly different times, due to the slit moving up as the camera panned right. But the car was going even faster yet, moving further right, ahead of the shutter in the frame, so it leans the other way. This classic picture is responsible for us imagining speeding wheels as being slanted ovals, at least in cartoon drawings. The narrow open slit can provide a very short (fast) exposure, but it takes much longer for this slit to travel across the frame. Shutters are much faster today, but the fastest motion can still be an issue. See the HSS flash picture of the grinding disk on next page.
But, flash pictures can only expose the full frame after the shutter is fully open. The front curtain takes a certain time to travel to the fully open position (and the rear curtain the same time to close again). So this curtain opening time (technically plus the brief flash duration time) is the duration of the Maximum Flash Sync Speed. Faster shutter speeds (shorter durations) will have already started closing earlier, blocking the flash exposure from the covered portions of the sensor. The curtain speed is WHY flash is typically limited to around 1/200 second (5 milliseconds) shutter speed on focal plane shutters today. Modern shutters became faster, flash sync was 1/60 second until about 1980. But materials are better today, and another speedup is that it is faster for shutters to move vertically down the sensor short dimension instead of horizontally across the long dimension.
Electronic shutters (like in compact cameras, simply timed by the CCD sensor chip turning on or off) are not limited about sync speed. Their flash can work at any speed, however most do still specify something like about 1/500 second maximum (fast cuts off the flash exposure of the tiny flash too). Iris shutters (in lenses of larger cameras) can sync flash at 1/500 second, or really any faster speed too, however if really fast, their opening and closing speeds (opening from the center) can expose the center area more than the corners.
Shutter speeds creating the Maximum Sync Speed: (semantics: a slower shutter SPEED is a longer shutter DURATION.) The focal plane shutter can be left open for any long exposure, minutes even if using Bulb shutter setting. (Here comes the tricky part). But the shortest time that the shutter can be actually fully open for flash (none of the full frame is blocked by either curtain) is greater than zero... just enough for the flash, meaning before the second curtain starts closing immediately just as the first curtain reaches fully open. The flash can be fired during that instant that both curtains were fully open (so that the flash can expose the full frame). This near zero time fully open case makes the exposure duration be the opening curtain travel time (to be evened out by the closing curtain travel time, making the exposure sum be one curtains travel time to fully open). This also makes one curtains travel time be equal to the Maximum Sync Speed by definition (becomes obvious after you think about it... this shortest travel duration is the maximum shutter speed that still has an instant of fully open shutter). So faster moving shutter mechanisms is what also creates faster Maximum Sync Speeds, being the maximum speed that the shutter curtain can be fully opened for flash. Modern shutters travel the short dimension vertically down (or up) the frame to be faster than travel across the longer dimension of the frame.
Shutter speeds faster than maximum sync speed (shorter shutter duration) : Faster shutter speeds start closing the second curtain before the first curtain is fully open.. The rear curtain closing is following behind the front curtain opening. The frame is never fully open. This exposes only a narrow open slit width (i.e., the timing between the two curtains is the exposure time, which is: duration = slit width / velocity). Flash cannot work then, since the frame is never fully open to pass it. The overall two curtain travel movement takes longer than the exposure from the narrow slit, but the actual exposure time is the time the open slit is over any area (the overall travel movement is always a constant travel time).
Advantage of focal plane shutters: The same remarkable precision mechanism moves both curtains, creating and moving a virtual narrow slit of opening across the frame at a fixed speed, to provide precise fast exposures under that slit. There are really good reasons better cameras provide the focal plane shutter.
Downsides of focal plane shutters
The flash is often relatively nearly instantaneous, but it has to be synced to trigger when the shutter is fully open. For focal plane shutters, the fastest shutter speed at which the full frame is all fully open at any one instant (to allow instantaneous flash to fully cover the entire frame), is called the Maximum Sync Speed. The Nikon spec charts in rear of camera manuals call it Maximum Flash Sync Speed, and I often tend to say Maximum Shutter Sync Speed (all the same thing, sync of the flash to the fully open shutter). Flash sync is not possible if the focal plane shutter speed is faster than this limit (because some of the frame would be covered by the closing second rear shutter, and so unexposed, causing a dark band in picture). Some cameras use leaf shutters in each lens, or electronic shutters in some camera sensor chips, and these can sync faster than focal plane shutters (because there is always a fully open instant).
The Maximum Sync Speed for a focal plane shutter relates to how fast the curtain can move across the frame, to the Fully Open position. The flash has to wait for the fully open position, which determines Maximum Sync Speed, because if faster, the second rear curtain has already began to close.
In the 1960s, the focal plane shutter maximum sync speed was 1/60 second. Maybe 1/80 second in the 1980s. Today, shutters are faster, and 1/200 second (5 ms) is common today. A few models can do 1/250 second (4 ms) or 1/320 second (3.125 ms). Full frame cameras must travel half again farther than the APC-C sensors, but the vertical shutter travel today covers 1/3 less distance than horizontal. Full frame covering 24 mm in 3.125 ms travels 7.7 meters/second. Cropped APC-S covering 16 mm in 5 ms travels 3.2 meters/second. There is no Minimum shutter speed — any slower shutter speed can always be used with flash. However most camera automatons arbitrarily force about 1/60 second Minimum Shutter Speed With Flash in camera A or P or Auto modes (E2 menu on some Nikon models). This is not about sync or exposure, it is simply a concern about hand holding slow shutter speeds. Slow Sync or Rear Curtain Sync modes will bypass that limit to allow any slow shutter speed actually metered.)
The Nikon specs clearly say Maximum Sync is still 1/250 second on their faster shutters, but some of these models provide a mode to allow 1/320 second sync, warning it might suffer reduced range of the flash. My guess is this is not about sync, but means the fast shutter could cut off slow flashes, like some large studio flash at low power, or speedlights at maximum power (but any speedlight power level lower than maximum should not be any issue). The speedlight T.1 flash duration at maximum will about 3x longer than T.5 (typically around 1/300 second). Maximum sync speed is normally not a speed problem (at normal ISO and apertures, when the ambient is not contributing much), because the flash is faster, so maximum sync speed is perfectly fine. The speed concern is about daylight level bright ambient, unable to use faster shutter speed.
Even in camera Manual mode, the camera will not allow you to choose these fast shutter speeds if it knows the flash is present (if flash is on the hot shoe for communication). But a PC sync cord will not know any flash is present, and this example above used a PC sync cord (flash not in hot shoe), to prevent communication, to fool the camera into allowing any shutter speed. There is no reason to do that, other than to show the problem here. An APS-C sensor crop exposed on a full frame camera may crop off a small dark band, so it might get away with a slightly faster shutter speed than if shooting full frame.
Historically in the distant past, the only way to get around this sync problem with a faster shutter was to use a special flashbulb (named FP sync, for focal plane) that burns slower and longer to stay fully illuminated much longer, perhaps 1/30 second delay to peak, and then 1/30 second duration at half power points, for the full shutter travel time — more like continuous light for the longer shutter duration. HSS flash is the same similar solution today, a flash duration lasting as long as the shutter travel time (next page). This longer duration simulates continuous light, which has no sync requirement.
Flash bulbs had guide numbers on the package too. For shutters that were not focal plane, they could use faster shutter speeds, but which cut off their longer light duration, so their guide numbers were typically maximums for 1/30 second shutter, and sometimes also a couple of stops less light at 1/250 second.
The shutter speed limit of Maximum Sync Speed is really no big deal for flash indoors. The flash is fast itself, so in dim ambient light (where we need flash), it matters less what shutter speed is, the flash is faster. The dim ambient is too dim to blur motion seen by a slow shutter, and indoors (if low ISO), we can open the aperture as wide as we wish without concern about overexposing the ambient. But this maximum sync speed becomes a much bigger problem when using fill flash in bright sun (because overwhelming bright sun cannot be ignored). The speedlight is extremely fast in dim light with no ambient to blur anything, but the bright sun is continuous, not fast.
The Sunny 16 Rule says at ISO 200, exposure in bright sunlight is 1/200 second at f/16. Bright sun does not vary, so this is the norm we expect. And we pretty much have to correctly expose the daylight scene. We cannot ignore the sun like we can ignore dim indoor illumination. Continuous light (like sunlight) has no shutter sync requirement, but when we add a flash for fill, now there is one. The topic of fill flash in sunlight is covered in Part 4.
With flash in sunlight, we cannot use faster equivalent exposures, like 1/400 second at f/11, or 1/800 at f/8 because the focal plane shutter's maximum sync speed is 1/200 or 1/250. That means, if using flash in bright sun at ISO 200, the camera exposure WILL NECESSARILY BE around 1/200 second at f/16 (Sunny 16, but cannot exceed sync speed). The f/16 requires a lot of flash power, but otherwise this works fine, unless you just craved to use f/2.8 out there. Camera P mode knows all about this, and has ability to set both shutter speed AND aperture, and so it is a good choice for fill flash in bright sun. But if you use camera A mode, and set f/4 out there without thinking, the camera will just fuss warning HI at you, until you set near f/16 so it can work in bright sun (at the limited sync speed).
Wishful thinking, but our dream is that if somehow we could increase shutter speed, we could open the aperture, for reduced depth of field, or to an equivalent exposure for daylight which lets the regular flash mode work at lower power level, without affecting either exposure. Or, a faster shutter speed could help the flash to "overpower" the sun, reducing the sun without affecting the flash — if we could, but we cannot (in these bright sun cases.). We are up against the maximum shutter sync speed wall. FWIW, using lower ISO, or using a Neutral Density filter, can allow a wider aperture in that case, but that's all it does (maximum shutter sync speed is still enforced). But these affect both the flash and the sun equally, so these do not change the balance between flash power and sun. We can only use more flash power to affect the balance, and then exposing the higher flash power might end up at f/32, which does decrease the sun that needs f/16, but we still need Maximum shutter speed sync. Flash in bright sun is a special case.
Flash sync speed is really only an issue in bright sun. It is unimportant in dimmer light, where we need flash. So we might as well get used to Maximum Shutter Sync Speed, because it has always been this way, since the first Leica 35 mm camera in 1925 (focal plane shutter). The first Nikon F (1959) sync'd flash at 1/60 second. The situation is better today than it has ever been before.
One workaround in sunlight is to underexpose the sunlight ambient by a couple of stops, so the flash can stop the motion without the ambient blurring it again. Flash becomes main light instead of fill. Sports action likes this, it highlights the subject better anyway. The speedlight is faster than the shutter, and reducing the sun's exposure causes less blurring of action. And speedlights are the way High Speed Photography is done.
A couple of confusion factors which do not change the basics:
There are mechanical focal plane shutters, and there are electronic shutters.
If we had the electronic CCD chip shutter (no mechanical motion involved), then these can sync flash at the fastest shutter speed. The CCD sensor is simply electronically enabled and disabled, which is also used as a shutter, even like at 1/4000 second. However, the faster shutter speeds could be fast enough to truncate the longer flash duration, reducing effective light from the full power flash (but speedlights become even much faster than the shutter, at lower power levels, in more dim light so the ambient won't blur anything).
The Nikon D40, D50, D70 cameras did use the CCD electronic shutter, but if these models recognize the flash is present, they still limit the maximum shutter sync speed to 1/500 second. However, with these electronic shutters, if you can break the communication to fool the camera so that it does not recognize a flash is present (for example, if you use a PC sync cord), then the camera firmware will not limit shutter speed at all. You can use any faster shutter second then (only on these models with this electronic shutter). The problem is not about syncing the electronic flash, the focal plane problem is about opening the shutter wide, to get the flash through it.
Note that just because these CCD situations might sync the flash with a fast electronic shutter, it does not mean there is no effect on the flash. A fast shutter surely does truncate the longer full power flash duration and reduce the useful flash exposure. However, lower speedlight power levels are much faster than any shutter, not likely affected.
It is difficult to define how to measure actual flash duration. Flash is a fast pulse which then decays relatively slowly, trailing off to zero, and it is hard to agree when it effectively finishes. When does the gradual trail-off stop being effective? (10%? 5%? 1%?) The standard method for published flash duration is called t.5, which measures the time that the flash is stronger than 50% of its peak intensity. This is an engineering convention, convenient for engineers, but is not a photographer's convention. Because 50% intensity is only one stop down, and is still rather bright, so photographically, this means the t.5 specification number is actually about three times faster than we realistically see in our pictures — the useful 10% limit of the flash (called t.1, measuring 90% of the power) is about 3 times longer than the conventional 50% spec number for duration. BUT the definition of this limit of usability is really difficult to specify. 10% is not much to a photographer, less than our smallest setting change of 1/3 stop, but 50% is a full stop.
Camera speedlight units are a big exception, and are "different", being extremely fast at lower power levels (the name "speedlight"). Their 1/32 power level may have an actual duration of 1/20,000 second, because speedights reduce power by truncating the flash duration. This curve of the truncated pulse has steep sides going up and down, and so t.5 really hardly applies, EXCEPT when at their full power level when t.5 and t.1 are probably the same (just saying, the truncated lower power speed specs are more unequivocally precise regarding what actually happens). But otherwise, speaking of the speedlight full power level (and studio lights), then in general, a t.1 time is more meaningful for photographers, which duration is mathematically about three times slower than the t.5 stated by the spec, and so the full power duration may approach normal maximum shutter sync speeds closer than we may realize. If that may be not be clear, please see the actual description at High Speed Flash.
The subject of this page. Flash is limited by a Maximum Shutter Sync Speed, typically around 1/200 second maximum. Start at the top of this page. :)
These focal plane cameras often offer a FP High Speed Sync mode (HSS) which in fact does allow any shutter speed faster than the maximum sync speed. This is actually a matching feature in both the camera and the flash unit, which drastically changes the flash behavior, but the camera must select it, and trigger it slightly earlier (HSS is a "system", flash and camera, offered in top end models). HSS is NOT High Speed Flash, actually, the extreme opposite, being continuous light with zero speed stopping capability. HSS is just High Speed Sync, meaning it allows faster shutter speed to be used with a flash, but which is NOT high speed speedight flash.
Auto FP mode is Nikons menu name to enable HSS mode. HSS is not actually the same thing as "flash", but HSS mode does offer one way to increase shutter speed with flash in bright sun.
HSS flash mode is a big subject, continued on next page.